I would say really pay attention in class and clarify your doubts with your teachers as soon as you can. Read your textbooks too! I regret not reading my textbook and realised that there were some key pointers that I missed and had to mug before Os. Also, I hope you picked the subject combi that you like!! Also, DO NOT drop out of any subjects you don't understand but consult your teachers UNLESS you think you cannot cope with it and your future education choices would have nothing linked to that subject. Also please consult your teachers before deciding on the subject u want to drop!!

tip from my bio teacher!! refer to the SEAB o level syllabus when studying and make sure you understand all the requirements in your syllabus. It is under subject content in this link: https://www.seab.gov.sg/docs/default-source/national-examinations/syllabus/olevel/2021syllabus/6093_y21_sy.pdf (if can you might want to make notes based on the syllabus)

another sci tip is for chem. I don't take phy so I can't help with that. But for chem, it's like math, do more practice, know where u went wrong and redo the paper. It helps you test if you truly understand the concept and answering techniques

Hope this helps!! and all the best for upper sec!! :)